The invention relates to a friction stir welding apparatus for joining workpieces and to a method for joining workpieces by means of a friction stir welding process.
The basic principles of friction stir welding processes are previously known, for example, from WO 93/10935 A1.
FIG. 1 shows in schematic representation the flow of a friction stir welding process according to the prior art. Contrary to the conventional friction stir welding processes, the friction stir welding uses a wear-resistant tool, comprising a welding pin 3 and a shoulder 4, to produce an integrally bonded connection. The tool which is used is here contoured such that solid material can be sheared in the contact region and stirred together. The plasticization process is aided by the influence of the generated frictional heat. The process temperatures are typically in the realm of the recrystallization temperature of the base material, so that the parts to be joined are not fused.
The friction stir welding tool has three primary tasks to fulfill (Mishra & Mahoney 2007, “Friction Stir Welding and Processing”-1st. Edition; ASM International 2007; ISBN 087170840X), these are: (1) heat regeneration by friction and shearing; (2) integrally bonded connection of the workpieces (parts to be joined) by stirring of the plasticized material; and (3) sealing of the top side of the seam by the shoulder.
During the joining process, the welding pin plunges fully into the joining region and is responsible for the process flow along the joining surface (butt surface, connecting surface) and for the generation of the material bond.
The task of the shoulder 4 primarily consists in, from the top side of the seam, closing off the joining zone 8. To this end, the tool or the shoulder surface is pressed throughout the joining process with a high contact force, generally at a setting angle of 2° to 4°, piercingly onto the surface of the workpieces. As a result of the relatively large bearing surface, the shoulder 4 is capable of, on the one hand, protecting the still warm weld metal from air access and, on the other hand, to prevent excessive escape of the material and thus the compression and consolidation of the sheared material. Furthermore, the shoulder 4, similarly to the welding pin 3, contributes to the heat generation and material shearing at regions close to the surface.
U.S. Pat. No. 6,264,088 B1 discloses a tool concept in which the welding pin can rotate independently of the shoulder.
Through the use of such a tool with stationary shoulder, various advantages are afforded to the user in relation to a conventional tool concept. These can include the following: (1) a clean, scale-free seam surface is formed, which seam surface, in terms of its appearance, resembles the unaffected base material; (2) the thermal and mechanical influence of the base material is almost homogenous in the direction of depth of the seam; and (3) new fields of application are opened up for friction stir welding, in which regard a T-joint weld seam (wedge seam) should be mentioned as an example.
However, the necessary anti-twist protection, in particular, which must be realized by an auxiliary element, often a bracket, has a negative impact. This bracket connects the tool housing to a fixed part of the machine. With this auxiliary device, an automatic tool change, as is customary in modern metal-working machines, carries the risk of a collision. As a rule, a prior removal of the tool attachment is therefore necessary. A further drawback can be seen in the fact that the contact force of the stationary shoulder must be applied via the drives of the machine.